Rosins have heretofore been employed in a diversity of applications including, inter alia, paper size, tackifier, rubber emulsifier, coatings, printing ink and so on. Recently, from the standpoints of safety and environmental protection, much attention has been focused on the technology of providing rosins in aqueous emulsion forms. Particularly in the field of sizing agents, improvements have been sought in aqueous rosin emulsions.
Cellulosic fiber products such as paper, cardboard, wood fiber board, etc. are generally manufactured in web-forming systems using size-containing aqueous dispersions of cellulosic fibers, and with regard to the sizing agents used for this purpose, saponified and fortified rosin sizes are commonly employed because they impart water resistance, resistance to bleeding, etc. to final sheets. However, such fortified rosin sizes cannot be properly used in high-temperature web-making processes. The recent rigorous control over effluent disposal calls for a closed papermaking system but the resulting increase in the temperature of papermaking water detracts from the effect of sizes. In the case of neutral paper which is lean in alum, i.e. aluminum sulfate, too, the effect of sizes is inadequate.
Therefore, in recent years, as substitutes for such fortified rosin sizes, rosin emulsion sizes which resolve many of the above problems have been used in preponderance in this field. Several methods are known for the production of such emulsion sizes. They include the method which comprises dispersing a fortified rosin in water in the presence of a suitable dispersing agent using a high-pressure shear homogenizer, the method in which a molten fortified rosin is subjected to phase inversion using a suitable dispersing agent and water, and the method comprising dissolving a fortified rosin in a solvent and emulsifying the solution with a dispersing agent and water. Among the dispersing agents which are generally used in the production of such emulsion sizes are sodium alkylsulfate, sodium alkylbenzenesulfonate, polyoxyethylene alkylphenyl ether, polyoxyethylene alkylphenyl ether sulfate, etc. However, emulsion sizes prepared with these known dispersing agents, which are invariably of low molecular weight, have the drawback of producing a copious foam in the process of web formation, although in sizing effect they are superior to said saponified and fortified rosin sizes. Moreover, these emulsion sizes are not satisfactory enough in sizing effect in high-temperature or high-pH systems.
More recently, much research and development work has been undertaken in the field of rosin emulsion sizes employing polymeric dispersing agents (Examined Japanese Patent Publication No. 48031/1984 and Unexamined Japanese Patent Publications No. 169898/1981 and No. 108796/1986). Rosin emulsion sizes obtainable with such polymeric dispersing agents are satisfactory in that they produce no appreciable foam but are still deficient in sizing effect in a high-temperature or high-pH web formation system.